1. Field of the Invention
The invention relates to the field of propulsion systems and, in particular, to a propulsion system for vertical and short takeoff and landing (V/STOL) aircraft.
2. Description of Related Art
The efficiency of a propulsion system for an aircraft increases as the exhaust velocity is reduced. Thus, during takeoff, landing and hovering, it is obvious that a helicopter, which provides a small incremental velocity to a large mass of air, is more efficient than a jet aircraft, which provides a large incremental velocity to a small mass of air. However, a helicopter, because of its very large diameter rotor, has a limited forward velocity, certainly not much over two hundred miles per hour. Thus, most V/STOL aircraft are compromises. For example, the AV-8A Harrier V/STOL aircraft utilizes a turbofan engine for both hover and cruise propulsion. As with a helicopter, the large fan provides significant thrust for vertical lift in hover, but its correspondingly large frontal area increases the drag of the aircraft and limits its maximum speed to subsonic speeds.
In U.S. Pat. No. 4,474,345 xe2x80x9cTandem Fan Series Flow VSTOL Propulsion Systemxe2x80x9d by R. G. Musgrove, a jet engine with a small fan, which is capable of providing supersonic performance, is modified to provide vertical lift. The basic engine fan is split to provide fore and aft fans connected by means of a common drive shaft. The fans are centrally mounted in a duct located within the aircraft along its longitudinal axis. In normal wing borne flight (herein after referred to as normal flight), the fans operate in series with the fan exhaust mixing with the turbine exhaust and exiting through a nozzle located at the rear of the aircraft. In the vertical mode of operation, a diverter is positioned downstream of the forward fan and is movable to a position for diverting the exhaust from the forward fan downward relative to the longitudinal axis of the aircraft, while simultaneously opening an auxiliary inlet for permitting the introduction of air to the aft fan. An aft diverter is located in the nozzle which is also moveable to a position for diverting the exhaust from the aft fan and engine core downward. Thus, for vertical flight the diverters are actuated causing the exhaust from both fans and the core engine to be directed downward fore and aft of the center of gravity of the aircraft.
However, the tandem fan engine has less thrust in the vertical takeoff and landing mode of operation than it has in the normal flight mode of operation. The thrust is larger in cruise because airflow passes through both fans, and thus, the core is supplied with air that is raised to a higher-pressure level; whereas, in the vertical mode the core engine airflow passes through only the aft fan. Consequently, the tandem fan concept is not an efficient design for a V/STOL aircraft.
In U.S. Pat. No. 4,791,783 xe2x80x9cConvertible Aircraft Enginexe2x80x9d by R. E. Neitzel, a turbofan concept is disclosed for converting almost all the power used by the engine fan to shaft horsepower to drive a helicopter rotor. Guide vanes located on both sides of the outer portion of the engine fan can be actuated to block off airflow through the fan duct while still allowing airflow into the engine core. A gear mounted on the forward end of the fan shaft is coupled to a drive shaft, which in turn drives the rotor. Such a system provides maximum efficiency during takeoff and landing and also during normal flight. However, if high-speed flight is to be accomplished the rotor must be stopped (x-wing concept) or stopped and stored. The former concept severely limits the top speed of the aircraft, while the latter causes a severe weight penalty and requires a complex folding and storing system.
In U.S. Pat. No. 5,209,428 Propulsion System For A Vertical And short Takeoff And Landing Aircraft by P. M. Bevilaqua, et al. In detail, the invention comprises a turbofan engine mounted within the airframe having the fan face coupled to the inlet duct. The engine is a mixed flow type having fan, high-pressure compressor, combustion, turbine and exhaust nozzle sections. The turbine section includes a high-pressure turbine portion which drives the high-pressure compressor section and a low-pressure turbine portion which drives the fan section. The inner portion of the fan section is in front of the high-pressure compressor section and, thus, acts as a low-pressure compressor section. Since a first shaft to the low-pressure turbine portion connects the fan section and the high-pressure compressor is connected to the high-pressure turbine portion by a hollow shaft rotatably mounted about the first shaft, they are often referred to as spools. Thus, in the turbofan engine thus far described, it is referred to as a two-spool engine. Furthermore, the high-pressure compressor section, combustion section and high-pressure turbine portion are, collectively, referred to as the core or core engine.
In this type of engine, the turbine exhaust produces a significant portion of the total thrust thereof and, preferably, has a common fan and turbine exhaust nozzle section exiting (mixed flow) at the rear of the aircraft when operated in the normal flight mode. The exhaust nozzle section is designed to divert exhaust flow either horizontally for normal flight or vertically downward for takeoff and landing, and intermediate positions therebetween when transferring from vertical to horizontal flight and visa versa. A vertically mounted lift fan assembly, having a lift fan rotor, is positioned forward of the engine and connected by a drive shaft to the front of the engine fan. A clutch is mounted in the driveline between the lift fan assembly and engine for disconnecting the lift fan rotor from the engine. Power to drive the lift fan rotor is obtained by increasing the engine exhaust nozzle area (exhaust nozzle section exit cross-sectional area). This allows more power to be extracted from the turbine exhaust during V/STOL operation. The excess power is absorbed by the lift fan rotor, which is xe2x80x9cclutched inxe2x80x9d during takeoff and landing and the transition to and from normal flight. By doing so, the operating point of the engine is shifted so that more power is applied to the lift fan rotor, which is more efficient at these lower speeds. The lift fan exhaust duct assembly is equipped with a vectoring system to deflect the thrust from a vertical direction in vertical flight to an aft vectoring direction during transition to and from normal flight. After transitioning to horizontal flight, the operating point of the engine is returned to its normal cruise mode of operation, which is more efficient at higher speeds.
To control the power extracted from the low-pressure turbine section, a mechanism is provided for varying the exhaust nozzle exit cross-sectional area. Depending on the particular design of the turbofan, it may be desirable to add one or more additional turbines to the low-pressure turbine section in order to extract the additional power. It is important to note that only the low-pressure turbine section will sense the reduction in back pressure caused by an increase in nozzle exit cross-sectional area; thus, the high-pressure turbine portion driving the high-pressure compressor section will sense little or no decrease in back pressure.
If the engine is operated during normal flight as a mixed flow turbofan engine, it can also be operated as a separate flow engine in the vertical flight mode of operation. This accomplished by blocking off the fan duct with a plurality of doors, which divert the fan section exhaust to roll control nozzle assemblies. The roll control nozzle assemblies consist of a pair of ducts, which connect to the fan duct aft of the fan section and extend outward therefrom; terminating in downward directed variable cross-sectional area roll control nozzles. Valves located in the ducts, at the fan section duct wall, open to admit fan exhaust to the individual roll control nozzle assemblies, which are differentially controlled to develop roll control forces. Pitch axis control power for the aircraft in the vertical and transitional flight regimes is achieved by thrust modulation between the lift fan assembly and the engine core turbine exhaust. This is accomplished by variable inlet guide vanes located just upstream of the lift fan rotor and the variable cross-sectional area exhaust nozzle section. As the angle of the inlet guide vanes is varied, the power to drive the lift fan rotor is varied. Alternatively or concurrently, the lift fan assembly nozzle cross-sectional area may be varied to provide the same effect. Changing the thrust produced by the lift fan rotor requires that the power extracted by the low-pressure turbine portion to correspondingly change. This, of course, can be accomplished by changing the nozzle section exit cross-sectional area. While modulating the lift fan rotor and engine core exhaust thrust levels for pitch control, the sum of these thrusts remain essentially constant.
Thus, for takeoff the lift fan rotor is xe2x80x9cclutched inxe2x80x9d and the engine exhaust nozzle assembly is positioned to divert the exhaust downward. The nozzle section exit cross-sectional area is increased so that additional power is extracted from the turbine exhaust by the low-pressure turbine section and applied to the fan section and lift fan rotor. Therefore, the fan section to core airflow ratio (commonly called the engine bypass ratio) will have been increased significantly resulting in a higher thrust-to-horsepower specific ratio. After takeoff, the exhaust nozzle section is adjusted back to a position where the exhaust is directed along the longitudinal axis, which is accomplished slowly as the aircraft gains speed. The operating point of the engine is returned to its normal position, when the fan rotor is de-clutched, by decreasing the nozzle section cross-sectional area. The actual transition points, rates of nozzle cross-sectional area change and nozzle diversion angle change, etc. will vary with the design of the particular aircraft and engine used.
This propulsion system has been successfully flown in the single engine XF-35 Joint Strike Fighter aircraft and will be used in the production aircraft to be manufactured by the Lockheed Martin Corporation. However, the use of a lift fan limits its placement within the aircraft, primarily to the fuselage and is unsuitable for use in multi-engine aircraft with the propulsion systems mounted on wing mounted nacelles. In addition, in some applications the weight and/or xe2x80x9cspacexe2x80x9d penalty associated with the use of a lift fan can not be tolerated in a single engine aircraft.
Thus, it is a primary object of the subject invention to provide a propulsion system for an aircraft, which allows for its placement in wing mounted engine nacelles of a V/STOL aircraft.
It is a still further object of the subject invention to provide a propulsion system for mounting in wing mounted nacelles of a V/STOL aircraft having improved efficiency in both the takeoff and landing flight modes.
An additional object of the subject invention is to provide a propulsion system for mounting in wing mounted nacelles of a V/STOL aircraft that is very efficient at high speeds and which has sufficient thrust during landing and takeoff.
The invention is a propulsion system for a V/STOL aircraft. In detail, the invention includes turbofan engine comprising: a fan section having a variable pitch fan; a compressor section; a combustion section, and a turbine section. The turbine section includes a low-pressure turbine portion coupled to and driving the fan section and a high-pressure turbine portion coupled to and driving the compressor section. Since a first shaft to the low-pressure turbine portion connects the fan section and the high-pressure compressor is connected to the high-pressure turbine portion by a hollow shaft rotatably mounted about the first shaft, they are often referred to as spools. Thus, in the turbofan engine thus far described, it is referred to as a two-spool engine. Furthermore, the high-pressure compressor section, combustion section and high-pressure turbine portion are, collectively, referred to as the core or core engine. In this type of engine, the turbine exhaust produces a significant portion of the total thrust thereof and, preferably, has a common fan and turbine exhaust nozzle section exiting (mixed flow) at the rear of the aircraft when operated in the normal flight mode.
The engine has a selectable operating point wherein a portion of the power generatable by the low-pressure turbine at a selected operating power setting is extracted to drive said fan section. A turbine exhaust duct directs the turbine section exhaust gases. A first angular shaped nozzle section coincident with the turbine exhaust duct directs exhaust from the fan section. The rear of the first nozzle section terminates in a rotatable nozzle and extends beyond the turbine exhaust duct and can be rotated from a horizontal position, wherein the rearward exhaust from the fan section as well as the first nozzle section is directed downward. A second exhaust nozzle section is mounted to the first angular shaped nozzle section between the fan section and compressor section. It is movable from a horizontal position directing a portion of the fan section exhaust rearward of the aircraft to a vertical position directing the fan section exhaust downward from the aircraft.
A roll control system is incorporated including first and second ducts having first ends coupled to the first nozzle section and second ends terminating in ports in the underside of the one of the wings of the aircraft. First control valves mounted in the first ends of the first and second ducts. Second control valves are mounted in the second ends of the first and second cuts. Thus when the first and second control valves are opened exhausts from the fan section can be diverted from the first annular shaped nozzle section and the second valves can be modulated providing role control.
Additionally, a mechanism is incorporated to shift the selectable operating point of the engine to a second operating point at the selected power setting increasing the power extracted by the low-pressure turbine portion of the turbine section. This mechanism is preferably a diverter valve assembly mounted in the first exhaust nozzle section aft of the first and second ducts of the roll control system. When actuated, this diverter valve assembly prevents fan section exhaust exiting the first nozzle section through the rotatable exhaust nozzle at the end thereof. This decreases pressure within the turbine exhaust duct. In addition, the rotatable exhaust nozzle of the first nozzle section can includes the capacity to increase its cross-sectional area, which also reduces pressure in the turbine exhaust duct. The engine may incorporate both mechanisms.
Both mechanisms will reduce the backpressure at the low-pressure turbine portion of the turbine section. Thus the power extracted by the low-pressure turbine portion and applied to the fan section can be increased without changing the selected power setting and the pitch of the blades of the variable pitch fan section can be increased to absorb the increased power.
In the V/STOL mode, pitch control can be accomplished by modulating the position of the diverter valves or the cross-sectional area of the rotatable exhaust nozzle of the first nozzle section with a simultaneous change in the pitch of the fan blades. That is for a pitch up, the backpressure on the low-pressure turbine section is reduced and the pitch of the fan blades is increased increasing the pressure so that more thrust to generated by the second nozzle section. For pitch down, the back-pressure in the low-pressure turbine section is increased, reducing the power transferred to the fan section with a simultaneous reduction in pitch of the fan blades. This causes more downward thrust from the rotatable exhaust nozzle of the first nozzle section and less thrust from the second nozzle section.
When the propulsion system is mounted within wing mounted nacelles on a multi-engine aircraft they function in a similar manner except for the roll control system. The roll control system in this application includes a first duct having a first end coupled to the first nozzle section of the right turbofan propulsion system and a second end terminating in a port in the underside of the left wing of the aircraft. The first duct includes first and second control valves mounted in the first and second ends, respectively. A second duct is included having a first end coupled to the first nozzle section of the left turbofan propulsion system and a second end terminating in a port in the underside of the right wing of the aircraft. The second duct includes third and fourth control valves mounted in the first and second ends, respectively. With this configuration, should one engine fail, horizontal attitude can still be maintained in the V/STOL condition.
For vertical takeoff, the engine is stared and the diverter valves are closed and roll control valves are opened and the second nozzle sections are rotated downward as well as the rotatable nozzles of the first nozzle section. The fan blade pitch is increased. This causes the engine core to transfer more power to the fan section. As the thrust is increased, the aircraft will take off vertically. After sufficient altitude is achieved, simultaneously, the second nozzle section is slowly rotated from the vertically downward position to the horizontal position; the end of the second nozzle section is rotated from the vertically downward position; the diverter valves are slowly opened; and the fan blade pitch is slowly decreased. This causes the engine operating point to shift and the aircraft transitions to horizontal flight. After sufficient horizontal speed is achieved, closing all the valves can deactivate the roll control system.
Transition back to the vertical flight mode is accomplished by essentially reversing the procedure. Simultaneously: the second nozzle section is slowly rotated from the horizontally downward position to the vertical position; the rotatable nozzle of the first nozzle section is rotated from the horizontal position to the vertically downward position; the diverter valves are slowly closed; the roll control valves are opened; and the fan pitch is increased. This causes the engine operating point to shift and the aircraft transitions to vertical flight. After horizontal speed is stopped, power can be reduced allowing the aircraft to descend and land.
The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description in connection with the accompanying drawings in which the presently preferred embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawings are for purposes of illustration and description only and are not intended as a definition of the limits of the invention.